The present invention relates to a system and a method for controlling a lean-burn internal combustion engine, and more particularly, to estimating instantaneous NOx conversion efficiency of the LNT.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream catalytic converter. The LNT stores exhaust components, such as oxygen and NOx, during lean operation. When the LNT is filled to a predetermined capacity, stored exhaust gas constituents are typically reduced and released (purged) by switching to rich or stoichiometric operation, i.e., by increasing the ratio of fuel to air and thereby increasing the amount of reductant such as hydrocarbon (HC) and carbon monoxide (CO) present in the exhaust gas mixture entering the LNT. Once the purge is completed, lean operation resumes again.
Prior art teaches monitoring the amount of NOx stored in the LNT as a result of lean operation, and, once that amount predetermined value, purging the LNT. Typically, the amount of NOx stored is determined based on the LNT NOx storage efficiency and the amount of NOx generated by the engine.
The inventors herein have recognized that the LNT efficiency may vary widely with changes in the LNT temperature, overall storage capacity, space velocity, etc. This variability makes it very difficult to obtain an accurate estimate of the LNT NOx storage efficiency, which may cause inaccuracies in the estimate of the total amount of NOx stored in the LNT. If the amount of NOx stored in the LNT since the last purge is underestimated, lean operation may continue after the LNT is full, and result in tailpipe NOx emissions. On the other hand, if the amount of NOx is overestimated, fuel economy may be reduced due to unnecessary purging.
The inventors have further recognized that a more accurate estimate of the amount of NOx stored in the LNT as a result of lean operation can be obtained by evaluating the LNT NOx storage efficiency as a function of available LNT NOx storage capacity. In that case, the efficiency would be directly related to the number of available storage cites in the LNT, and not subject to variability in the overall LNT NOx storage capacity or operating conditions such as temperature or space velocity.
In view of the above, a system and a method are provided for obtaining a highly accurate estimate of the amount of NOx stored in the LNT as a result of lean operation.
In accordance with the present invention, method of determining an amount of NOx stored in an exhaust gas aftertreatment device coupled downstream of a lean-burn internal combustion engine, includes: estimating NOx storage efficiency of the device based on a percent NOx capacity filled; and calculating the amount of NOx stored in the device based on said estimated NOx storage efficiency of the device.
An advantage of the present invention is improved emission control, due to the fact that a more accurate estimate of the amount of NOx stored in the LNT will prevent overfilling of the LNT.
Another advantage of the present invention is improved fuel economy, since a more accurate estimate of the amount of NOx stored in the LNT will prevent unnecessary purging.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.